Electroless nickel plating composition and method for its preparation and use

ABSTRACT

An aqueous bath for the electroless plating of nickel is disclosed, in which nickel is utilized in the form of the tris(hydrazine carboxylato-N 2 ,O) nickelate(1-) complex. Methods for preparing such a bath and for plating various metal surfaces in the bath are also disclosed.

FIELD OF THE INVENTION

This invention relates to an improved electroless nickel platingcomposition and to a method for applying a nickel layer to a metalsurface.

BACKGROUND OF THE INVENTION

The increasing sophistication of today's semiconductor chips requirescorresponding technological advancements in the packaging of such chips.Ceramic chip carriers often make use of alumina-based substrates uponwhich discrete areas of multilayer metallization have been bonded.Generally, the metallization comprises (a) a base metallization layerbonded to the ceramic substrate, (b) a layer of nickel bonded to thebase layer, and (c) a layer of gold bonded to the nickel layer. The basemetallization layer is often formed of a refractory metal such astungsten which may be screen-printed onto the substrate surface.

The nickel layer enhances wire bonding while also providing a goodthermal expansion match between the tungsten and gold layers. While thislayer may be applied by either electrolytic or electroless platingtechniques, electroless plating is increasingly being used because ofits ability to apply very uniform layers of nickel to complex, nonplanarsurfaces, such as chip carrier surfaces having patterned contact holesand vias for electrical interconnection.

In these applications, it is often desirable that the nickel films bechemically pure. Unfortunately, the most common electroless nickelplating baths known in the art, which employ hypophosphite, borohydrideor amine boranes as the chemical reducing agent, all deposit nickelfilms that contain about 1%-15% by weight phosphorous or boron asimpurities. These impurities reduce the electrical conductivity of thenickel film; make it brittle; and cause it to be a relatively inferiorsurface for soldering.

It is known from the prior art that pure nickel films can be depositedfrom electroless plating baths in which hydrazine is used as thereducing agent. An exemplary electroless nickel plating bath of thiskind is disclosed in W-D Haack's U.S. Pat. No. 3,915,716, and includeshydrazine, ammonia, monoethanolamine or diphosphate as a complexingagent, and carbonates or orthophosphates to buffer the bath at a pHbetween 11 and 12. Furthermore, in U.S. Pat. No. 3,198,659 and in "ThinNickel Films by Hydrazine Autocatalytic Reduction", ElectrochemicalTechnology, 1, 38-42 (1963), D. J. Levy discloses nickel platingcompositions which include a nickel salt, sodium hydroxide, one ofseveral complexing agents, and hydrazine as a reducing agent. In "ThickNickel Deposits of High Purity by Electroless Methods", Plating, 54,385-390 (1967), J. Dini et al. disclose a nickel plating compositionwhich can contain nickel acetate, glycolic acid, tetrasodium EDTA, andhydrazine. V. M. Gershov et al. disclose a nickel-plating bathcontaining nickel sulphate, hydrazine sulphate and monoethanolamine, inTemperature Activation of Chemical Nickel- Plating in HydrazineSolutions, Russian Engineering Journal, Volume 53, No. 10, pp. 73-74.

Unfortunately, the attributes of these electroless plating compositionsare accompanied by several disadvantages. For example, the use of thesebaths to plate refractory metals often requires activation of the metalsurface, such activation generally preceded by a series of complicated,rigorous cleaning steps.

Furthermore, some of the electroless plating baths of the prior art arehighly unstable under temperature conditions necessary for platingnickel onto various substrates.

Moreover, the plating rates achieved by using some of these baths isvery low, less than about 3 microns per hour, even at platingtemperatures as high as 95° C. Such plating rates, along with theability to form only very thin nickel films, greatly diminishes thevalue of such baths in many commercial applications.

Other electroless plating compositions, such as those disclosed in theGershov et al. reference mentioned above, are able to achieve highplating rates only when used at very high temperatures (100° C.-200°C.).

It is therefore an object of the present invention to provide a highlystable electroless nickel plating composition.

It is another object to provide a plating composition which can be usedto apply chemically pure nickel to a metal substrate.

It is a further object to provide a nickel plating composition whichallows nickel to be plated directly upon refractory metal surfaceswithout prior activation of the surfaces.

It is still another object of the present invention to provide areliable and practical method of electrolessly applying nickel to ametal purface at high plating rates and at moderate plating bathtemperatures.

Another object of the present invention is to provide a method forpreparing such an electroless nickel plating composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of nickel thickness as a functionof plating time in an electroless plating bath maintained at varioustemperatures, wherein the pH is 11.73.

FIG. 2 is a graphical representation of nickel thickness as a functionof plating time in an electroless plating bath maintained at varioustemperatures wherein the pH is 11.47.

DESCRIPTION OF THE INVENTION

The nickel plating baths of this invention are characterized by thepresence of the tris(hydrazine carboxylato-N²,O) nickelate(1-) complex.This complex, which hereinafter may be referred to as the "nickelcomplex" or by its formula, Ni(N₂ H₃ COO)⁻ ₃, functions as a source ofnickel for plating. Its Chemical Abstracts Compound Registry Number is51911-36-5. The complex may be preformed and added separately to theplating bath, but is preferably formed, in situ, by the addition to thebath of precursor compounds which react to form the complex.

This nickel complex is described, for example, in two successivearticles by A. Braibanti et al., "Tris(hydrazine carboxylato-N,O) Metal(II) Anions in the Solid State and in Solution I. Potassium Salts" and"Tris(hydrazine carboxylato-N,O) Metal (II) Anions in the Solid Stateand in Solution. II. Spectroscopic and Magnetic Investigation", Ric.Sci. 36, 1153-1156 and 1156-60 (1966); Chemical Abstracts: Vol. 66,108869s (1967) and Vol. 67, 58924d (1967), respectively.

The amount of nickel complex present in the plating bath depends onseveral variables, including the desired plating rates and thicknesses,as well as the chemical and physical characteristics of the substratebeing plated, e.g., the degree to which the substrate surface has beenactivated. The bath contains at least about 0.01 mole of nickel complexper liter of solution, and substantially all of the nickel (i.e.,substantially greater than 99%) is present in the form of the complex.Since higher levels of the nickel complex often result in higher platingrates, preferred embodiments of this invention call for at least about0.1 mole of the complex per liter of bath solution.

The plating bath of this invention employs hydrazine as the reducingagent for the nickel complex. Hydrazine may be added to the bath in theform of hydrazine itself or as a hydrazine hydrate. The hydrazine, i.e.,that which is in addition to the hydrazine forming a part of the nickelcomplex, is present in an amount sufficient to reduce substantially allof the complex to nickel metal on a substrate surface. The molar amountof hydrazine reducing agent should be at least equal to the molar amountof nickel present in the bath, as further described in the exampleswhich follow.

The bath should have a pH in the range of about 10 to about 13, and morepreferably, in the range of about 11 to about 12. The most preferablerange is about 11.4 to about 11.8. In general, a higher pH results inhigher plating rates, but also tends to lower bath stability.

Furthermore, preferred embodiments of this invention include bufferingthe pH. The buffer replaces OH⁻ ions which are consumed in the platingreaction, thereby serving to maintain a relatively constant depositionrate durig plating. Several buffers are suitable for maintaining the pHin the above-described range. One of these is a mixture of phosphatesalts, e.g., dibasic potassium phosphate (K₂ HPO₄) and tribasicpotassium phosphate (K₃ PO₄). Another suitable buffer is a mixture ofphosphate and hydroxide salts, e.g., a mixture of K₂ HPO₄ and KOH. Therequired level of buffer may easily be determined by monitoring the bathpH. The examples which follow describe exemplary quantities of bufferused in specific plating compositions.

A suitable temperature range for the bath during plating is about 70° C.to about 80° C., although bath temperatures as low as about 60° C. andas high as about 90° C. are also possible. When plating onto unactivatedrefractory surfaces, an especially preferred bath temperature is about75° C. to 80° C., as shown in the examples which follow.

The electroless plating bath of the present invention may be prepared inseveral ways. For example, the nickel complex may be preformed and addedin the form of a salt to a quantity of water prior to addition of theother ingredients; or the salt of the complex could be later added to anaqueous solution containing the other ingredients. An example of a saltuseful for this technique is the potassium salt of the complex. Theamount of nickel in the complex would be used to determine the requiredamounts of the other ingredients which are discussed below.

In preferred embodiments of the present invention, the nickel complex isformed in situ by reacting aqueous hydrazine carboxylate with a nickelsalt to form a solution containing the complex, free hydrazine, and freehydrazine carboxylate. Hydrazine carboxylate, discussed further below,is a potent complexing agent for nickelous ion. The preferred nickelsalt is nickel acetate, although nickel chloride and nickel nitrate arealso suitable. If sodium, rather than potassium, is the cationic speciesin the bath (as described below), nickel sulfate or nickel perchlorateare suitable sources of nickel because sodium sulfate and sodiumperchlorate are highly soluble in water, but potassium sulfate andpotassium perchlorate are not. The nickel salt may be dissolvedseparately in water and then added to a solution containing thehydrazine carboxylate.

The hydrazine carboxylate may be formed by reacting stoichiometricportions of bicarbonate and hydrazine. Potassium bicarbonate ispreferred, although sodium bicarbonate and lithium bicarbonate mightalso be suitable. The preferred preparation technique employs potassiumbicarbonate and hydrazine hydrate as reactants. The particular requiredamount of these materials depends on plating conditions and nickelquantity, and falls within the guidelines outlined below and furtherillustrated in the examples which follow.

Hydrazine carboxylate is advantageously present in the bath in an amountsufficient to stabilize the nickel complex, i.e., to substantiallyprevent its decomposition. The appropriate amount of hydrazinecarboxylate may be calculated from Equation (1) and the correspondingequilibrium constant K₁ : ##EQU1##

The equilibrium constant K₁ is very large, greater than 10¹⁴, for theplating temperatures contemplated here. Those skilled in the artunderstand that if hydrazine carboxylate is present in excess, thensubstantially all of the Ni⁺² is held in solution as the nickel complex.Thus, the particular amount of hydrazine carboxylate which will preventdecomposition of the nickel complex may be calculated for a given set ofconditions without undue experimentation. In general, at least about 4moles of hydrazine carboxylate per mole of nickel is sufficient.

In preferred embodiments, a carbonate compound is also added to theplating composition in an amount sufficient to stabilize the hydrazinecarboxylate, i.e., to substantially prevent its decomposition. Suitablecarbonate compounds include sodium carbonate, lithium carbonate or, mostpreferably, potassium carbonate. The carbonate should be added to amixture containing the hydrazine carboxylate prior to the addition ofthe nickel salt.

The appropriate amount of carbonate may be calculated from Equation (2)and the corresponding equilibrium constant K₂ : ##EQU2##

The equilibrium constant K₂ has a value of approximately 10⁻² for theplating temperatures contemplated herein. It is evident that for aparticular hydroxyl ion concentration, the amount of carbonate whichwill prevent the spontaneous decomposition of the hydrazine carboxylatemay be readily calculated. Generally, approximately 0.4 mol/L to 0.6mol/L of carbonate should be present in a plating bath which operates ata temperature of about 70° C.-80° C. and a pH of about 11.4 to 11.8.Those skilled in the art understand that according to the equilibriumexpression K₂, a change made in the concentration of one species must becompensated for by a change in the concentration of another species,since K₂ is constant at any given temperature.

Further details and exemplary preparations of these baths are providedin the examples below.

The plating bath contains at least one cationic species in an amountsufficient to neutralize the negative charges of the anionic species inthe bath, such as the carbonate and hydrazine carboxylate anions. Thecationic species is added to the bath in the form of a salt of one ofthe other components, e.g., a carbonate or bicarbonate salt.Illustrative cationic species include potassium, sodium, and lithium.Potassium salts are generally preferred for the present inventionbecause of their relatively high solubility in the plating bath. Thus,the bath might very well contain potassium in the form of potassiumcarbonate (K₂ CO₃), potassium bicarbonate (KHCO₃), potassium hydroxide(KOH), and the like.

As further described below, the use of the nickel complex in combinationwith the other ingredients in the present composition allows highplating rates and good quality deposition of nickel on both activatedand unactivated metal substrates. Furthermore, the plating bath isextremely stable. In order to characterize this stability, it shouldfirst be noted that fine metal particles form spontaneously at somefinite rate in all electroless plating baths. Because these particlesare catalytic sites on which further metal deposition will actively takeplace, they will eventually cause decomposition of the bath as all ofthe metal therein is plated onto the particles. In other electrolessnickel plating baths known in the art in which hydrazine is used as thereducing agent, the rate of formation of these fine particles is veryfast, and bath decomposition is complete in less than about 24 hours atoperating temperatures. In the present bath, the rate of formation ofthese fine particles is much slower, and the bath may be operated forperiods of four months or longer if the particles formed in it areperiodically removed, e.g., by filtering the bath about once every 24 to48 hours. Moreover, this stability can be maintained without theaddition of conventional stabilizing agents such as thiourea or heavymetal compounds. Use of these conventional stabilizing agents isunsatisfactory if pure nickel deposits are required because they alsointroduce undesirable impurities in the deposit, such as sulfur or leadatoms.

Another embodiment of the present invention is a method of electrolesslyapplying a layer of nickel to a metal substrate. Substrates which may beplated with nickel according to this method include refractory metalssuch as tungsten and molybdenum, as well as other metals that arenaturally catalytic to such deposition, such as iron, cobalt, copper,rhenium, palladium, platinum, and gold. An important feature of thismethod is its use in plating nickel on unactivated tungsten ormolybdenum, since activation is usually required to promote plating onthese metals.

Prior to plating, the substrate surface generally is cleaned bywell-known methods, such as the use of a mild soap solution and/ordegreaser material, followed by rinsing in deionized water and thendrying. Alternatively, a metallized ceramic substrate may be cleaned byheating in hydrogen gas or in gas mixtures containing hydrogen and aninert gas such as argon or nitrogen, for about 30 minutes at about 100°C.

In those instances in which activation of the substrate prior to platingis desired, activation may be accomplished by any suitable method. Forexample, the substrate may be washed with mild soap in an ultrasonicbath, followed by rinsing and then soaking in deionized water in theultrasonic bath. The substrate may then be immersed in a solutioncontaining the activator, e.g., a solution of palladium chloride towhich has been added sufficient hydrochloride to bring the pH to about1.7. After the activation treatment, the substrate may be rinsed andthen soaked again in deionized water in the ultrasonic bath. Thosehaving ordinary skill in the art appreciate that other activationmethods would also be suitable.

In practicing this method, the plating bath described above is containedin a vessel made of a material inert to the plating chemicals, e.g., avessel of glass or of a plastic such as polyprppylene. The plating bathis heated to maintain the temperature between about 70° C. and 80° C.Stirring of the bath provides both chemical homogeneity and uniformplating solution temperatures. The substrate surface is maintained inmotion, e.g., by rotation, to dislodge gas bubbles which can adhere tothe substrate surface and decrease the amount of plating composition incontact with the surface, thereby reducing plating efficiency.

Reactants consumed during the deposition of nickel, such as hydrazine,nickel ion, and hydroxyl ion, are replenished from time to time. Forexample, the hydrazine content may be periodically measured by titrationand then restored to its original value by adding more of the hydrazinecompound. The nickel ion concentration may be determined by colorimetryor by titration and then restored to its original value by adding moreof the nickel salt. Furthermore, the addition of an alkali metalhydroxide such as potassium hydroxide maintains the pH at its originalvalue.

If the appearance of the plated nickel is rough in texture and/or brownin color, the plated substrate may be heat-treated for about 20-40minutes at approximately 600° C. to about 700° C. in an atmosphere of10% hydrogen in argon. Such a treatment results in theelectroless-plated nickel having a bright, shiny metallic grayappearance.

Plating rates when the presently-described method is employed depend ona variety of factors, including the amount of nickelate hydrazinecomplex used, pH, plating temperature, and the like. Plating rates ashigh as 22 microns per hour have been achieved.

As mentioned above, the plating baths of the present invention exhibit ahigh level of stability whether in use or in storage. For example, manyof these bath compositions may be effective in plating nickel for atleast about 4 months. This stability is an especially desirableattribute in commercial plating operations wherein parts such assemiconductor chip carriers must be plated in quantity on a continuousproduction line with very little "downtime".

The scope of the present invention also includes the application of asecond layer of nickel or another suitable metal by electroplatingtechniques, the details of which are known in the art.

EXAMPLES

The following examples are provided to more fully describe the variousembodiments of this invention. It is intended that these examples beconsidered as illustrative of the invention, rather than limiting whatis otherwise disclosed and claimed herein.

The following chemical formulae may be used in the examples:

potassium bicarbonate--KHCO₃

hydrazine hydrate--N₂ H₄.H₂ O

potassium carbonate--K₂ CO₃

nickel chloride--NiCl₂.6H₂ O

potassium phosphate--K₂ HPO₄

potassium hydroxide--KOH

EXAMPLE 1

This example describes the preparation of a plating bath according tothe present invention.

An aqueous solution for the electroless deposition of nickel, having afinal volume of 3.0 liters, contained 135.2 grams KHCO₃ (0.45 mol/L),97.7 grams N₂ H₄.H₂ O (0.65 mol/L), 207.3 grams K₂ CO₃ (0.50 mol/L),72.4 grams of 98.4% NiCl₂.66H₂ O (0.10 mol/L), 261.3 grams K₂ HPO₄ (0.50mol/L), and 48.8 grams KOH (0.25 mol/L).

The KHCO₃ and N₂ H₄.H₂ O were dissolved in about 1 liter of deionizedwater, and the solution was stirred for about 4 hours at roomtemperature in order to allow the formation of hydrazine carboxylate.The K₂ CO₃ was then added to the solution and dissolved in it. TheNiCl₂.6H₂ O was dissolved separately in about 100 mL water, and thissecond solution was added to the first solution. The mixture was stirredfor approximately 5 minutes. The K₂ HOP₄ and the KOH were dissolvedseparately in about 500 mL water with the substantial evolution of heat.This third solution was then cooled to room temperature and added to thefirst solution. The resulting solution was then diluted to its finalvolume of 3.0 liters. The pH at room temperature was 11.7.

EXAMPLE 2

This bath was prepared according to the present invention by the use ofa mixed nickel salt containing carbonate and hydroxide anions. Anaqueous solution having a final volume of about 3.0 liters contained135.6 grams 99.7% KHCO₃ (0.45 mol/L), 97.7 grams N₂ H₄.H₂ O (0.65mol/L), 193.6 grams K₂ CO₃ (0.47 mol/L), 261.3 grams K₂ HPO₄ (0.50mol/L), 22.7 grams of 87.2% KOH (0.12 mol/L), and 38.31 grams of basicnickelous carbonate, 46.0% by weight nickel, having an approximatecomposition: NiCO₃.2Ni(OH)₂.4H₂ O (0.10 mol/L Ni).

The KHCO₃ and N₂ H₄.H₂ O were dissolved in about 1.5 liters of deionizedwater, and the solution was stirred for about 4 hours at roomtemperature to allow the formation of hydrazine carboxylate. The K₂ CO₃was then added to the solution and dissolved therein. The K₂ HPO₄ andthe KOH were dissolved separately in about 500 mL of water with thesubstantial evolution of heat. This second solution was cooled to roomtemperature and then added to the first solution. The basic nickelouscarbonate solid was then added. The mixture was stirred for about 16hours at room temperature. The nickelous carbonate solid had dissolvedafter this time. The mixture was then diluted to its final volume of 3liters, and exhibited a pH of 11.7.

EXAMPLE 3

In an alternative preparation, the basic nickelous carbonate compounddescribed in Example 2 was first dissolved in an acid, such as aqueousorthophosphoric acid. This procedure shortens the time required for bathpreparation as compared to the procedure used in Example 2. An aqueoussolution having a final volume of about 3.0 liters contained 135.7 gramsKHCO₃ (0.45 mol/L), 97.7 grams N₂ H₄.H₂ O (0.65 mol/L), 207.3 grams K₂CO₃ (0.50 mol/L), 86.1 grams of 85.2% (0.25 mol/L), 38.3 grams basicnickelous carbonate, 46.0% by weight nickel, having an approximatecomposition NiCO₃.2Ni(OH)₂.4H₂ O (0.10 mol/L Ni), 130.7 grams K₂ HPO₄(0.25 mol/L), and 106.6 grams of 87.2% KOH (0.55 mol/L).

The KHCO₃ and N₂ H₄.H₂ O were dissolved in about 1.0 liter of deionizedwater, and the solution was stirred for about 60 minutes at roomtemperature to allow the formation of hydrazine carboxylate. The K₂ CO₃was then added to the solution and dissolved in it. The K₂ HPO₄ and KOHwere dissolved separately in about 500 mL of water with the substantialevolution of heat. This second solution was cooled to room temperatureand then added to the first solution. A third solution was then preparedwhich contained the H₃ PO₄ diluted with about 100 mL of water. The basicnickelous carbonate was added to this third solution, promptlydissolving therein. The third solution was then added dropwise to thefirst solution. A green precipitate formed transiently as each drop ofthe third solution struck the first, but this solid material immediatelydissolved, yielding a clear blue solution. The solution was then dilutedto a final volume of 3.0 liters, and had a pH at room temperature ofabout 11.6. The total elapsed time in preparing this solution was muchshorter than the time required in Example 2.

Examples 4-7 describe methods for plating metal substrates according tothe present invention.

EXAMPLE 4

A solution was prepared with a composition described in Example 1,except that the amount of 86% KOH was increased to 58.7 grams (0.30mol/L). This solution had a pH of 11.7, and was used to deposit nickelonto 24 tungsten-metallized ceramic chip carriers. The chip carriersurfaces were unactivated in any way, except that 59 days previously,they had been heated inhydrogen gas at 1000° C. for 30 minutes to cleanthem and to reduce oxides on the tungsten surfaces. They had been storedin air at room temperature since that time.

The plating composition was heated to a temperature of about 80° C.±1°and maintained at that temperature. The 24 parts were tumblebarrel-plated in the bath with a Sterling Systems miniature tumblebarrel. Nickel plating commenced immediately upon immersion of the partsin the solution, as was evident from the immediate appearance ofvigorous bubbling as nitrogen gas evolved. The plating process Wasinterrupted at elapsed times of 2.5, 5, 7.5. 10, and 12.5 minutes, with4 parts being removed from the barrel on each occasion. The plating ofthe remaining 4 parts was stopped at 15 minutes. Visual inspectionshowed that nickel was plated on each part uniformly over all tungstensurfaces and nowhere else. The parts were then rinsed in deionized waterand dried.

Nickel thicknesses were measured by X-ray fluorescence. The mean andstandard deviation for each set of 4 chip carriers at each plating timeis shown in Table 1 below:

                  TABLE 1                                                         ______________________________________                                        Plating Time (min.)                                                                          Ni Thickness (microns)                                         ______________________________________                                        2.5            0.44 ± 0.06                                                 5.0            1.01 ± 0.06                                                 7.5            1.44 ± 0.03                                                 10.0           1.88 ± 0.11                                                 12.5           2.38 ± 0.09                                                 15.0            2.70 ± 0.08.                                               ______________________________________                                    

These values are also plotted in FIG. 1, which will be described below.

EXAMPLE 5

The temperature of the bath in Example 4 was reduced to 76°±1° C., andthe experiment was repeated with slightly different plating times.Again, as in Example 4, nickel deposition commenced immediately. Thefollowing results were obtained:

                  TABLE 2                                                         ______________________________________                                        Plating Time (min.)                                                                          Ni Thickness (microns)                                         ______________________________________                                        2.5            0.21 ± 0.08                                                 5.0            0.69 ± 0.06                                                 7.5            0.93 ± 0.03                                                 10.0           1.41 ± 0.13                                                 15.0           2.03 ± 0.09                                                 20.0            2.68 ± 0.08.                                               ______________________________________                                    

These values are also plotted in FIG. 1.

EXAMPLE 6

The temperature of the bath in Example 4 was reduced to 72°±1° C., andthe experiment was repeated (with slightly different plating times).Nickel deposition appeared to commence only after several minutes. Thefollowing results were obtained:

                  TABLE 3                                                         ______________________________________                                        Plating Time (min.)                                                                          Ni Thickness (microns)                                         ______________________________________                                        5.0            0.27 ± 0.04                                                 7.5            0.45 ± 0.12                                                 10.0           0.84 ± 0.06                                                 15.0           1.44 ± 0.15                                                 20.0           1.84 ± 0.08                                                 25.0            2.40 ± 0.05.                                               ______________________________________                                    

These values are also plotted in FIG. 1.

EXAMPLE 7

The temperature of the bath used in Example 4 was reduced to 66°±1° C.,and the experiment was repeated again. Nickel deposition did not beginfor about 18 minutes. The following values were obtained:

                  TABLE 4                                                         ______________________________________                                        Plating Time (min.)                                                                          Ni Thickness (microns)                                         ______________________________________                                        5.0            0.00 ± 0.04                                                 10.0           0.00 ± 0.03                                                 15.0           0.00 ± 0.03                                                 20.0           0.24 ± 0.23                                                 30.0           0.72 ± 0.47                                                 40.0            1.91 ± 0.22.                                               ______________________________________                                    

These values are also plotted in FIG. 1.

FIG. 1, which plots values obtained from Examples 4-7, demonstrates thatthe plating of nickel on unactivated tungsten substrates may be achievedat a variety of temperatures. At lower temperatures, e.g., 66° C., therewas a noticeable time lag before deposition began. However, at 80° C.,plating began immediately.

In Examples 8-11, a plating bath identical to that of Examples 4-7 wasemployed, except that the pH was decreased by decreasing the amount ofKOH used.

EXAMPLE 8

A solution was prepared as described above for Examples 4-7, except thatthe amount of 86% KOH added was decreased to 39.1 grams (0.20 mol/L).The pH of this solution was 11.5 at room temperature. The bath washeated to and maintained at a temperature of 80°±1° C., and plating wascarried out as in the previous examples with the following results:

                  TABLE 5                                                         ______________________________________                                        Plating Time (min.)                                                                          Ni Thickness (microns)                                         ______________________________________                                        5.0            0.42 ± 0.16                                                 7.5            0.89 ± 0.04                                                 10.0           1.14 ± 0.12                                                 15.0           1.70 ± 0.06                                                 20.0           2.07 ± 0.07                                                 25.0            2.11 ± 0.66.                                               ______________________________________                                    

These results are plotted in FIG. 2.

EXAMPLE 9

The temperature in the bath of Example 8 was reduced to 76°±1° C., andthe experiment was repeated, with results as shown in Table 6:

                  TABLE 6                                                         ______________________________________                                        Plating Time (min.)                                                                          Ni Thickness (microns)                                         ______________________________________                                        10.0           0.67 ± 0.08                                                 15.0           1.07 ± 0.11                                                 20.0           1.71 ± 0.12                                                 25.0           2.03 ± 0.07                                                 35.0           2.79 ± 0.07                                                 40.0            2.99 ± 0.26.                                               ______________________________________                                    

These values are also plotted in FIG. 2.

EXAMPLE 10

The temperature in the bath of Examples 8 and 9 was reduced to 72°±1°C., and the experiment was repeated. Results are shown in Table 7:

                  TABLE 7                                                         ______________________________________                                        Plating Time (min.)                                                                          Ni Thickness (microns)                                         ______________________________________                                        10.0           0.00 ± 0.09                                                 20.0           0.32 ± 0.14                                                 25.0           0.32 ± 0.30                                                 35.0           1.18 ± 0.64                                                 45.0           2.03 ± 0.18                                                 55.0            2.06 ± 1.13.                                               ______________________________________                                    

These values are also plotted in FIG. 2.

FIG. 2 depicts the effect of decreasing the pH of the plating bath. Asin FIG. 1, the deposition rate generally increased with increasingtemperature. Furthermore, a comparison with FIG. 1 demonstrates thathigher plating rates are also achieved by raising the pH of the bath.

The data shown in FIGS. 1 and 2 also demonstrates that a time lag whichoccurs when plating at lower temperatures is not present at the highertemperatures, i.e., at or above 76° C.

If plating at the lower temperatures is desired, activation of thesurface prior to plating by well-known methods, e.g., use of palladiumchloride, would eliminate the time lag.

EXAMPLE 11

This examples demonstrates that a plating bath operated under theconditions described above deposits nickel on tungsten-plated chipcarriers even when the tungsten surface has not previously beenheat-treated in hydrogen gas.

A plating bath having a composition as in Example 1 was prepared. Its pHat room temperature was 11.6. The bath was heated to and maintained at75°±1° C. 100 ceramic chip carriers of the type described in previousexamples were used. However, these chip carriers were not heattreated inhydrogen gas beforehand. After about 60 minutes, the carriers wereremoved from the solution and visually examined. There was no sign ofnickel deposition. After another 15 minutes of immersion, they wereagain removed and examined. Sporadic nickel deposition was observed.After another 30 minutes of immersion, the chip carriers were againremoved and examined. Nickel deposition was present everywhere on everytungsten-metallized region of each chip carrier. The nickel thickness on40 carriers selected at random was about 1.8±0.5 microns. Sincesubstantially all of this nickel accumulated during the last 30 minutesof immersion, the rate of nickel deposition, once begun, apparently wasgreater than about 3.5 microns/hour, a rate comparable to those in theprevious examples.

While the invention has been described with respect to preferredembodiments, it will be apparent to those of ordinary skill in the artthat certain modifications may be made without departing from the spiritand scope of the invention and, therefore, it is intended that theforegoing disclosure be limited only by the appended claims.

What is claimed is:
 1. An aqueous bath for the electroless plating of nickel, comprising tris(hydrazine carboxylato-N²,O) nickelate(1-) complex and an amount of hydrazine sufficient to reduce substantially all of said complex to nickel metal on a substrate surface, wherein said bath has a pH of about 10 to about
 13. 2. The bath of claim 1 further comprising hydrazine carboxylate and carbonate.
 3. The bath of claim 2 wherein hydrazine carboxylate is present in an amount sufficient to stabilize the tris(hydrazine carboxylate-N²,O) nickelate(1-) complex, and carbonate is present in an amount sufficient to stabilize the hydrazine carboxylate.
 4. The bath of claim 3 comprising at least about 0.1 mole of said complex per liter of solution.
 5. The bath of claim 4 wherein the pH is about 11.4-11.8.
 6. The bath of claim 5 further comprising a phosphate salt mixture as a buffer.
 7. The bath of claim 5 further comprising a mixture of phosphate and hydroxide salts as a buffer.
 8. A method for the electroless deposition of nickel which comprises immersion of a metal substrate into an aqueous plating bath comprising the tris(hydrazine carboxylate-N²,O) nickelate complex and hydrazine, said bath having a pH of about 10 to about
 13. 9. The method of claim 8 wherein the bath further comprises hydrazine carboxylate and carbonate.
 10. A method according to claim S wherein the substrate metal is selected from the group consisting of tungsten and molybdenum and is nonactivated prior to the electroless deposition.
 11. The method of claim 8 wherein the bath contains at least about 0.1 mole of said complex per liter of solution.
 12. The method of claim 11 wherein the pH is maintained at about 11.4-11.8.
 13. The method of claim 12 wherein the bath temperature is about 70° C. to about 80° C.
 14. A method of preparing an electroless plating bath for applying a layer of nickel to a metal surface, comprising:reacting aqueous hydrazine carboxylate with a nickel salt to form a solution comprising tris(hydrazine carboxylato-N²,O) nickelate(1-) complex, free hydrazine, and free hydrazine carboxylate.
 15. The method of claim 14 wherein the hydrazine carboxylate is formed by reacting potassium bicarbonate with hydrazine hydrate, said hydrazine carboxylate stabilized by the addition of a carbonate compound.
 16. The method of claim 14 wherein the nickel salt is nickel acetate.
 17. The method of claim 14 wherein the bath is maintained at a pH of about 10 to about
 13. 18. A method of preparing a plating bath for the electroless deposition of nickel on a metal surface, comprising dissolving hydrazine and a salt of tris(hydrazine carboxylato-N²,O) nickelate(1-) complex in an aqueous medium.
 19. The method of claim 18 wherein the bath also contains hydrazine carboxylate and carbonate, and is maintained at a pH of about 10 to 13 by the use of a buffer.
 20. The method of claim 19 wherein the buffer is a mixture of phosphate salts. 